This application describes a method of integrating pharrnacokinetic (PK) and viral phenotypic drug susceptibility (IC50) information to assist clinicians in choosing optimal regimens for salvage therapy of their treatment-experienced HIV-infected patients. PK parameters from multiple dual protease inhibitor (PI)-based or non-nucleoside analog (NNRTI) regimens were collected from the literature. PK models with built-in variability were created for each regimen and used to simulate 100 concentration-time curves for each regimen. This provides a wide range of trough levels that can be expected in the average patient population. The IC50 for a PI or NNRTI is corrected for plasma protein binding and compared with the range of simulated trough levels (Craig). In using this approach, called Probability Estimations, it is possible to determine the probability of achieving trough levels above the protein-binding corrected IC50 (PBICs0) for each drug. This method ranks each possible regimen based on the likelihood of achieving the desired plasma concentrations. This approach is directly applicable to the clinic, and does not require measurement of drug levels. The hypothesis is that integration of PK and phenotypic information maximizes the benefit of antiretroviral therapy in the management of treatment-experienced subjects. Viral drug resistance is a continuum; a certain fold-change in virus susceptibility does not necessitate a drug will be inactive and achievable drug concentrations in the patient must be considered. The specific aims of this application are: 1) to prospectively validate the Probability Estimations method by correlating the estimated probability of achieving PI trough concentrations above the PBIC95 for each participant at baseline with their subsequently measured actual IQ derived from an intensive pharmacokinetic evaluation at week 4 while receiving an antiretroviral regimen selected based on the Probability Estimations approach; and 2) to assess short-term virologic response in a prospective, 16-week "proof-of-concept" pilot study by enrolling 60 subjects with moderate-level drug resistance into two randomized arms. One arm will have their subsequent salvage regimen chosen using a phenotypie test for clinician guidance, the other arm will use the Probability Estimations approach that inherently includes phenotyping plus drug pharmacokinetics for clinician guidance. All subjects will undergo an intensive PK evaluation at week 4, and followed every 4 weeks for 16 weeks to assess short-term virologic response. The long-term objectives of this work are: (1) to allow expansion of these observations into a larger, well-controlled clinical trial with optimized sample size calculations based on the phase I study results generated here; and (2) to ultimately expand these findings into less treatment-experienced patients to demonstrate its widespread clinical applicability.